1. Field of the Invention
This invention relates to a method of thermal development, and more particularly relates to a method of thermal development of a thermally developable light-sensitive recording material using microwaves.
2. Description of the Prior Art
As compared with the conventional process of heating a light-sensitive layer through heat conduction or the like using a heater, the process of thermal development using microwaves has the advantage that the time elapsed from the application of an electric potential to an apparatus to reach a thermally developable temperature, i.e., the warming-up time of the apparatus, is extremely short. Therefore, the apparatus need not be left turned on resulting in a saving of electric power. Further, heating with microwaves eliminates the possibility of damage of a light-sensitive layer since heating without contact is possible.
A thermally developable recording material as described above usually has a structure in which a thermally developable light-sensitive layer is formed on a support, such as paper, a synthetic resin sheet, glass, etc., either directly on the support or on a subbing layer on the support. However, this recording material has the defect that, upon thermal development of a recording material with this structure by placing the recording material in a microwave field, as long as 4 to 5 minutes is required for enough heat to be generated to thermally develop the recording material since the microwave energy is scarcely absorbed by the recording material.
As one means to overcome this defect, a suggestion has been to provide a dielectric substance having a high dielectric constant and a high dielectric loss tangent in or on a support of a recording material or in a light-sensitive layer to thereby accelerate heating, as described in, e.g., Japanese patent publication No. 18,039/72. However, recent experiments which have now been conducted have shown that, even when a recording material containing such a dielectric substance is heated using microwaves, almost no effects were obtained.
That is, the penetration depth of the microwaves absorbed by a dielectric substance is generally high, and a support such as a paper, a polyethylene terephthalate sheet, soda glass or the like shows a penetration depth as high as 10 to 100 cm for microwaves of a frequency of 2450 MHz. While extremely effective heating is attained in heating a material having such a thickness, in heating a thin material (e.g., a thickness of about 100 .mu.) such as a light-sensitive material, microwaves are scarcely absorbed by a light-sensitive layer even when a substance having a high dielectric constant and a high dielectric loss tangent are used or added, thus failing to greatly increase the heating efficiency. The reason for this is that the heating efficiency of a dielectric substance in absorbing the energy of the microwaves increases as the dielectric constant and the dielectric loss tangent of the substance increases. A dielectric substance such as a paper, a polyethylene terephthalate sheet, soda glass or the like, however, generally has a low absorbing efficiency, and the depth (penetration depth) required to absorb one half of the energy of the microwaves by the dielectric substance is about 10 to 100 cm. Here, the absorbing efficiency for the microwaves per unit of depth is high with a shallow penetration depth of the microwaves. In a thin dielectric substance (e.g., a thickness of about 100 .mu.), the heating efficiency can be increased in any manner in which the dielectric constant and the dielectric loss tangent thereof can be increased such as using or adding a substance having a high dielectric constant and a high dielectric loss tangent. However, even this fails to greatly increase the heat efficiency because of low absorbing efficiency of the substance for the microwaves themselves.